This invention generally relates to brake control systems for wheeled vehicles and, more particularly, to brake torque control systems for limiting torque developed during braking of a wheeled vehicle, particularly commercial airplanes.
Brake systems of large commercial transport airplanes include pilot-operated brake controls coupled to hydraulically actuated brakes in the main landing wheels of the airplane. In simple outline, the pilot's brake pedals are coupled to a pressure metering valve that admits hydraulic fluid to the brakes at a pressure which varies in response to the manual braking force exerted by the pilot. Such airplanes typically further include an automatic braking system which, when armed by the pilot during a landing approach, automatically applies the brakes in a predetermined manner on touchdown so as to free the pilot to attend to other matters during landing of the airplane.
In addition to the basic elements described above, the braking system of a large commercial airplane typically includes an antiskid system that operates to automatically prevent skidding of the tires overriding the pilot's manual braking commands where necessary to prevent skidding. Antiskid systems are important since uncontrolled skidding of a large commercial airplane can result in wearing through of the landing wheel tires within a few seconds and may also result in loss of control over the airplane.
Such antiskid systems typically include a brake pressure relief valve, sometimes called an antiskid valve, interposed in the hydraulic pressure line of each brake. When the antiskid system senses a skid condition, an antiskid control signal causes the antiskid valve to open quickly and reduce brake pressure, thereby correcting the skid condition.
The antiskid system typically detects a skid condition by measuring the angular deceleration of the landing wheels. More specifically, a wheel speed transducer associated with each wheel provides a wheel speed signal, the time derivative of which provides a signal indicative of wheel deceleration. The antiskid system is responsive to the wheel deceleration signal to actuate the antiskid valve whenever the wheel deceleration signal exceeds a predetermined value. Examples of antiskid systems are disclosed in U.S. Pat. No. 4,078,845 to Amberg et al. and in U.S. Pat. No. 4,180,223 to Amberg, the contents of which are hereby incorporated by reference.
The present invention is addressed to a problem not adequately solved by conventional braking systems, including systems having antiskid capabilities as described above. Specifically, certain large commercial airplanes are being retrofitted with carbon brakes to replace steel brakes originally installed on the airplane. The steel brakes and the carbon brakes are similar in structure and generally include multiple interleaved stator and rotor brake discs. The rotor discs are affixed to and rotate with the landing wheel. The stator discs are nonrotating and are affixed to a stationary brake housing and axle assembly. A hydraulic actuator mechanism drives the stacked rotor and stator discs together to provide braking action. In the carbon brakes the stator and rotor discs are formed of a carbon-based material that can withstand higher braking temperatures than can steel brakes.
One advantage in switching to carbon brakes is a net reduction in airplane weight of as much as 1600 pounds. This reduction in weight is achieved with no penalty in brake performance. Additionally, as stated above, the ability of the carbon brakes to withstand higher braking temperatures renders them capable of absorbing larger amounts of heat generated by friction during braking. As a result, the carbon brakes can be applied longer and harder than steel brakes under the same conditions.
Carbon brakes, like steel brakes, exhibit a coefficient of friction that varies as a function of wheel speed, temperature, age, moisture and other factors. It is found, however, that carbon brakes exhibit a considerably greater range of variation in their coefficients of friction than do steel brakes. As a result, the coefficient of friction of the carbon brakes exceeds under certain circumstances the maximum coefficient of friction previously obtained with steel brakes. Accordingly, the brake torque obtained by application of carbon brakes with a particular hydraulic brake pressure is at times considerably greater than the maximum brake torque previously obtained with steel brakes. This poses certain structural load problems nonexistent with steel brakes, as discussed further below.
The brake torque developed at a landing wheel is transmitted through a torque equalizing assembly so as to be fully borne as a rearward translational load by the landing gear strut assembly. The landing gear was originally designed to accommodate the loads encountered with steel brakes, which, as stated above, fall within a narrower range than the loads experienced with carbon brakes. It has been found that under certain conditions, for example with a fully loaded airplane landing on a dry runway, the brake torque developed with carbon brakes may be sufficiently greater than that previously obtained with steel brakes as to result in stresses on the landing gear strut assembly which exceed the design load limits of the landing gear. With steel brakes, such high torque was never developed and the load limits of the strut assembly were therefore never exceeded. With the installation of the new carbon brakes, however, it becomes necessary to ensure that the brakes are not applied so as to exceed the load limits of the strut assembly. The antiskid system alone cannot provide this protection, since it is responsive to wheel speed rather than brake torque. Similarly, simply limiting the brake pressure is not adequate to prevent excessive torque because brake torque varies in a rather complex and not altogether predictable manner in response to varying brake pressure, and is additionally dependent on other factors such as the presence of water in the brakes.
Accordingly, it is the primary object and purpose of the present invention to provide a braking system that includes a brake torque control system which operates to limit the torque developed at a braked wheel of a vehicle. Although the present invention solves the particular problem described above with respect to the change to carbon brakes in commercial airplane landing wheel assemblies, it will be recognized that the invention may be of general applicability to other applications where it is desirable to limit the torque developed at a braked wheel.
It is also an object and purpose of the present invention to provide a simple and reliable brake torque control system that is compatible with and retrofittable onto a preexisting airplane braking system having an antiskid system as described above. In this regard, it is an object to provide an airplane braking system which includes both skid control and torque control systems which operate independently and in parallel to prevent skidding as well as excessive torque conditions at all times.
In the development of a torque control system to solve the landing gear load problem described above, it has been sought to provide a system which can be integrated with existing antiskid systems. Particularly, it has been sought to provide a system which prevents torque overshoot when, for example, an airplane comes out of a skid on ice and the torque rises rapidly as the braked wheels contact dry pavement. Under such conditions it is desirable to have the torque control system track the output of the antiskid system so as to anticipate and compensate for the sudden rise in torque that ordinarily results when the airplane comes out of a skid under the control of the antiskid system. Accordingly, it is another object to provide a torque control system which acts independently of the antiskid system to limit torque, and which yet also tracks the antiskid output to prevent torque overshoot as the airplane comes out of a skid.
To obtain optimum braking performance, it is desirable that the brakes be fully operable up to the load limits of the landing gear and yet also be fully responsive to the pilot's commands over this range. Previously knonw or readily apparent methods for limiting brake torque by conventional feedback control based on brake torque measurement are not satisfactory. Specifically, there has not been available previously a torque control system that is sufficiently responsive to rapid brake pressure application to prevent torque overshoot (development of torque greater than a predetermined maximum level) and which also provides a stable, damped control signal only within a narrow torque range at or near the load limit of the landing gear. Typically, prevention of torque overshoot has been obtained at the expense of diminished brake response sensitivity or at the expense of operating the system well below the actual torque limit so as to provide a safe margin of overshoot error.
For example, an airplane braking system having both antiskid and torque limiting systems is disclosed in U.S. Pat. No. 4,043,607 to Signorelli et al. The system of Signorelli et al. includes a torque limiting device for the purpose of preventing damage to the brakes from high torque conditions. In the system of Signorelli et al., a torque transducer provides a signal representative of measured brake torque to a servocontrol device which compares the measured torque signal with a reference signal. The value of the reference signal varies in response to the pilot's displacement of the brake pedal controls. Referring to the disclosure of Signorelli et al., and particularly FIG. 4 for example, actuation of the brakes is under the direct control of the pilot at torque levels less than a predetermined value and under feedback control of a servocontrol device 30 at torque levels above such predetermined value. More particularly, at low torque levels, signals representing the pilot's braking commands pass through a direct actuation device 32. When the torque exceeds the predetermined level, the pilot's command signals pass through the servocontrol devic 30 which regulates application of the brakes.
The direct actuation device 32 of Signorelli et al. includes a means for automatically limiting the rate of increase of brake pressure to a predetermined maximum rate. This prevents torque overshoot that might otherwise result from a sudden application of the brakes, but limits somewhat the responsiveness of the brakes to the pilot's manual controls. At higher torque levels, torque overshoot is prevented while braking is under the control of the servocontrol device 30 by regulating the application of brake pressure in response to a feedback signal representing actual measured torque such that the actual torque obtained is represented by predetermined torque functions, or laws, A and B, which are illustrated in FIG. 7 of Signorelli et al. The actual selection of law A or law B is made automatically by the servocontrol device 30 on the basis of the aircraft ground speed. Predetermined threshold, or maximum, values of the torque, indicated as Cca and Ccb in FIG. 7, limit the maximum brake pressure that may be applied under servocontrol. It will be apparent to one of ordinary skill, therefore, that torque overshoot is prevented in the system of Signorelli et al. by automatically limiting the rate of increase of brake pressure to a predetermined ramp function when braking is under the direct control of the pilot via the direct actuation device 32, and is prevented while braking is under the control of the servocontrol device 30 by reason of the feedback controlled application of the brakes. Thus, in both cases, torque overshoot is prevented to some extent by limiting the responsiveness of the braking system to the pilot's commands. It is, therefore, another object of the present invention to provide an improved torque control system that effectively operates to prevent substantial torque overshoot beyond a predetermined maximum torque without also limiting the rate of increase of brake pressure obtainable by direct command of the pilot.